The technological evolution in wind power generation is involving increasingly larger and more powerful machines which, in conjunction with taller towers, increase the output power generated by the wind. The development of taller towers has to be done according to different rules related to design criteria such as load, material strength and dynamic behavior as well as construction, transportation and installation conditions.
Up to 80 m, towers for wind power applications have been made basically using steel tubular sections. Taller towers involving high loads require more resistant structures. Due to transportation problems, bottom diameters larger than 4.5 m are not possible. This requires the steel plate thickness to be increased up to values that make the tower cost to be increased.
The use of concrete for constructing certain parts of wind turbines towers (particularly at the bottom of the tower) is a prominent alternative. Preliminary cost investigations have clearly shown that an improvement in tower costs can be achieved by substituting steel bottom portions by concrete bottom portions, specially in tower heights above 80 m. Moreover, despite the material used in the construction, the design of the concrete portions has to be carried out in accordance with the same rules that for steel portions, that is, loads, strength and mainly dynamic behavior. In order to complain with the dynamic behavior, thin-walled concrete portions have to be designed. Nevertheless, such a thin-walled concrete portion design leads to extra complexity in tower design.
Shear strength connections, such as shear studs or the like, are in general not valid in such applications due to geometrical complexity in its construction (steel reinforcement layout) and due to dimensional limitations of the tower wall.
WO2005015013 discloses a tower for a wind turbine comprising a concrete tower segment and a steel tower segment having an end portion embedded into the concrete tower segment. The steel tower segment within its embedded end portion comprises anchoring elements. Said anchoring elements project radially from an inner or an outer surface or both inner and outer surfaces of the wall of the steel tower segment. This however does not overcome the above mentioned problem of an easy steel-concrete integration in concrete thin-walled towers in wind turbine applications. This is because a very large concrete wall thickness is needed in the tower design disclosed in this document for withstanding horizontal load components resulting from such protruding elements.